One of the greatest demands for improvement in integrated circuit photoresists is in the area of thermal resistance. The reason is that post-baking of resists, after development, at temperatures of 250.degree. C. for up to 30 minutes, is commonly required in order to remove residual solvents in preparation for low vacuum ion implantation. Furthermore, the process of ion implantation itself often involves high temperature build-up. Present commercial resists, and particularly positive-working resists, tend to lack such thermal resistance. For example, positive-working resists comprising light-sensitive quinone-diazide sensitizer compounds in novolac resin binders suffer a loss of image integrity especially in the wall angles. That is, their image profiles tend to "slump" and they lose their line width control. This lack of resistance to heating occurs in spite of the presence of a recurring rigid ring structure (the phenylene group) as part of the polymer backbone in the novolac resin. Such rigid ring structures had been thought to contribute thermal resistance.
Resists have been described in the prior art literature wherein various synthetic polymers have been blended "to increase the strength" of the images. For example, styrene-maleic anhydride copolymers have been added to conventional light-sensitive layers, as described for example in U.S. Pat. No. 4,196,003, issued on 4/1/80. However, such a copolymer has been found to cause very slow development times which are unacceptable for practical use. A more active solvent will wash away the desired areas more rapidly but often at the expense of the areas that are intended to remain. Proper developer selectivity provides that development will occur in 30 sec, and that the loss after 30 sec of development in the remaining resist portions (which in the positive resist are unexposed), will be no more than 10%.
Still other polymers have been created with rigid ring structures as part of the backbone, but these polymers have not been as desirable as binders because they lack sufficient aqueous-base solubility when combined with the decomposition products of the exposed sensitizer.
Therefore, there has been a need prior to this invention for an aqueous base solvent-developable binder for photoresists that will withstand, after development, high temperature processing at 250.degree. C. with substantially no significant change or distortion in image profiles, and still provide a practical development time as discussed above.